usb/bootloader: cleanup detection of bootloader mode. add user-facing instructions for installing zadig on windows

lib/usb-connection.js

@@ -312,18 +312,24 @@ USB.Connection.prototype._waitUntilInBootloader = function() {
     var retry = () => {
       this._findBootedDeviceBySerialNumber(this.device.deviceDescriptor.iSerialNumber)
         // A  Tessel was found!
-        .then(function(bootedDevice) {
+        .then(bootedDevice => {
           // Make sure it's in the proper mode
           if (bootedDevice) {
             return resolve(bootedDevice);
           }
           // It didn't find it
-        }, function(err) {
+        }, error => {
           if (--retryCount > 0) {
             return setTimeout(retry, retryTimeout);
-          } else {
-            reject(err);
           }
+
+          if (process.platform.startsWith('win')) {
+            log.info('Placeholder for zadig installation instructions.');
+          }
+
+
+          // No tries left...
+          reject(error);
         });
     };
 
@@ -333,22 +339,25 @@ USB.Connection.prototype._waitUntilInBootloader = function() {
 
 // Looks through all attached USB devices for a Tessel in bootloader mode
 USB.Connection.prototype._findBootedDeviceBySerialNumber = function(serialNumber) {
-  return new Promise(function(resolve, reject) {
+  return new Promise((resolve, reject) => {
+    var found;
     // Fetch all attached USB devices
     if (usb) {
-      var list = usb.getDeviceList();
-
-      for (var i = 0; i < list.length; i++) {
-        var device = list[i];
-        // Make sure this is a Tessel
-        if ((device.deviceDescriptor.idVendor === TESSEL_VID) && (device.deviceDescriptor.idProduct === TESSEL_PID) && (device.deviceDescriptor.iSerialNumber === serialNumber)) {
-          // Make sure it's in bootloader mode
-          if (device.deviceDescriptor.bcdDevice >> 8 === 0) {
-            return resolve(device);
-          }
+      found = usb.getDeviceList().find(device => {
+        if ((device.deviceDescriptor.idVendor === TESSEL_VID) && 
+            (device.deviceDescriptor.idProduct === TESSEL_PID) && 
+            (device.deviceDescriptor.iSerialNumber === serialNumber) && 
+            (device.deviceDescriptor.bcdDevice >> 8 === 0)) {
+          // Shift bcdDevice bits rightward 8 bits to check that
+          // the device is in bootloader mode. 
+          return device;
         }
-      }
+      });
     }
+
+    if (found) {
+      return resolve(found);
+    } 
     return reject(new Error('No device found in bootloader mode'));
   });
 };